Rubber composition and process for production thereof

ABSTRACT

The present invention provides rubber compositions having an excellent transparency and rubber properties comprising a rubbery polymer and an inorganic filler, and relates to specific methods of producing said rubber. Thus, they are rubber compositions comprising 100 parts by weight of at least one rubbery polymer and 5-150 parts by weight of an inorganic filler per 100 parts by weight of said rubbery polymer, wherein the haze is 50% or less, and the inorganic bulking agents are dispersed so that the number of circular regions with a diameter of 0.5 μm or greater having no inorganic fillers is one or less and the number of circular regions with a diameter of 0.3 μm or greater having no inorganic fillers is three or less in an image of a 5 μm square of said rubber composition determined by a scanning probe microscope (SPM) and wherein the haze of said rubber composition having a thickness of 5 mm is 50% or less, and specific methods of producing them.

TECHNICAL FIELD

The present invention relates to rubber compositions having an excellenttransparency, excellent rubber properties, specifically compression-set(c-set) resistance and abrasion resistance, or a high hardness and anexcellent transparency, methods of producing them, and footwearcomprising said rubber compositions.

BACKGROUND ART

In various applications that require transparency and coloring,thermoplastic elastic rubber compositions in which an inorganic fillerhas been blended in a thermoplastic elastic body have conventionallybeen reported. For example, there are rubber compositions comprising athermoplastic elastic body and silica, rubber compositions comprising athermoplastic elastic body and titanium oxide, and the like.

Also, usually in the blending of vulcanized rubbers, when a closed-typemixer for example a Banbury mixer or an internal mixer is used to addand knead an inorganic filler and the like, temperature rises as soon asrubber mastication is started, and during the process of risingtemperature an inorganic filler etc. are added, and it reaches atemperature higher than 60° C. before the passage of 30 seconds at thelatest after the start of kneading.

When an open roll etc. is used, kneading has been performed at a highrubber temperature exceeding 60° C. in order to enhance rubberplasticity and facilitate kneading.

JP-A-62-137002 relates to shoe soles having an excellent coloring andabrasion resistance obtained by specifying silica and a silane couplingagent and mixing rubber with a thermoplastic resin, and describes thatcoloring may be effected without using carbon black.

JP-A-2000-236905 discloses a light-weight abrasion resistant solingmaterial comprising a high cis-polybutadiene, a styrene resin andsilica.

JP-A-2001-161404 describes shoe soles in which rubber and silica havebeen and the gravity thereof has been specified.

Rubber compositions using a thermoplastic elastic body indeed havetransparency and coloring to a certain degree, but they have problems ofpoor c-set and poor abrasion resistance, and poor oil resistance. Inrubber compositions for which such transparency and coloring arerequired, attaining balances between transparency and physicalproperties such as c-set and abrasion resistance while maintainingsufficient transparency have not been sufficient. On the other hand, inrubber compositions in which oil resistance has been improved byvulcanization of styrene butadiene rubber, there are problems that whenthe amount of inorganic fillers is reduced in order to enhancetransparency and coloring properties, the physical properties of thevulcanized rubber compositions such as c-set and abrasion resistancebecome decreased, and conversely when the amount of inorganic fillers isincreased, these physical properties may be enhanced to a certain degreebut transparency and coloring properties become decreased.

It is an object of the present invention to provide transparent rubbercompositions having an excellent transparency and coloring properties,and rubber properties such as c-set and abrasion resistance, methods ofproducing them, and footwear having excellent designing and functionalproperties using said transparent rubber compositions.

DISCLOSURE OF THE INVENTION

After intensive and extensive study on rubber compositions for whichtransparency is required in order to solve the above problems, it wasfound that the above objective can be attained by a specific haze valueand a specific dispersion state of the inorganic filler. Thus, it can beattained by providing the following rubber compositions.

Thus, the present invention relates to the following aspects:

(1) A rubber composition comprising 100 parts by weight of a rubberypolymer and 5-150 parts by weight of an inorganic filler per 100 partsby weight of said rubbery polymer, wherein the haze of said rubberycomposition having a thickness of 5 mm is 50% or less, and the inorganicfiller is dispersed so that the number of circular regions with adiameter of 0.5 μm having no inorganic fillers is one or less and thenumber of circular regions with a diameter of 0.3 μm having no inorganicfillers is three or less in an image of a 5 μm square of said rubbercomposition determined by a scanning probe microscope (SPM).

(2) The rubbery composition according to (1) wherein said rubberypolymer comprises 5-80% by weight of the rubbery polymer component (A)having a refractive index of 1.535 or greater and less than 1.600 and20-95% by weight of the rubbery polymer component (B) having arefractive index of 1.400 or greater and less than 1.530.

(3) The rubber composition according to (2) wherein the difference inrefractive index between component (A) and component (B) is 0.010 orgreater.

(4) The rubber composition according to (2) or (3) wherein component (A)is a copolymer of a vinyl aromatic compound and a conjugated dienecompound in which the amount of the vinyl aromatic compound bound is26-90% by weight, and component (B) is a diene polymer in which theamount of the vinyl aromatic compound bound is 25% by weight or less,and the haze of said rubber composition having a thickness of 5 mm is37% or less.

(5) The rubber composition according to any one of (2) to (4) whereinthe amount of the vinyl aromatic compound bound in the copolymer ofcomponent (A) is greater than that in the polymer of component (B) by25% by weight or more.

(6) The rubber composition according to any one of (2) to (5) whereinthe acetone extract in each of component (A) and component (B) is 2% byweight or less.

(7) The rubber composition according to any one of (2) to (6) whereincomponent (B) is a polybutadiene in which the amount of 1,4-cis bond is80% by weight or more.

(8) The rubber composition according to any one of (1) to (7) whereinthe inorganic filler is a silica filler having a specific surface areaof 100 m²/g or more.

(9) The rubber composition according to any one of (1) to (8) whereinthe inorganic filler is a dry silica having a mean primary particlediameter of 30 nm or smaller.

(10) A method of producing a rubber composition comprising 100 parts byweight of a rubbery polymer and 5-150 parts by weight of an inorganicfiller per 100 parts by weight of said rubbery polymer, wherein the hazeof said rubber composition having a thickness of 5 mm is 50% or less,and the inorganic filler is dispersed so that the number of circularregions with a diameter of 0.5 μm having no inorganic fillers is one orless and the number of circular regions with a diameter of 0.3 μm havingno inorganic fillers is three or less in an image of a 5 μm square ofsaid rubbery composition determined by a scanning probe microscope(SPM), said method comprising kneading said rubbery polymer and saidinorganic filler for at least 30 seconds or longer per run of kneadingat a rubber temperature of 0-60° C.

(11) The method of producing a rubber composition according to (10)wherein the rubber composition comprises 5-80% by weight of the rubberypolymer component (A) having a refractive index of 1.535 or greater andless than 1.600 and 20-95% by weight of the rubbery polymer component(B) having a refractive index of 1.400 or greater and less than 1.530,and said rubbery polymer and the inorganic filler are kneaded for atleast 30 seconds or longer per run of kneading at a rubbery temperatureof 10-50° C.

(12) A footwear having excellent designing and functional propertiesusing the rubber composition according to any one of (1) to (9).

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a photograph of an image of a 5 μm square of the rubbercomposition in Example 1 determined by a SPM, and

FIG. 2 is a photograph of an image of a 5 μm square of the rubbercomposition in Comparative Example 1 determined by a SPM. In thesephotographs, hard silica particles are indicated in black. In the rubbercomposition of Example 1, silica is uniformly dispersed, and there areno regions with a diameter of 0.3 μm or more having no inorganicfillers. In the rubber composition of Comparative Example 1, silica isnot uniformly dispersed, and there can be seen many regions having noinorganic fillers.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention relates to a rubber composition comprising 100parts by weight of a rubbery polymer and 5-150 parts by weight of aninorganic filler per 100 parts by weight of said rubbery polymer,wherein the haze of said rubber composition having a thickness of 5 mmis 50% or less, and the inorganic filler is dispersed so that the numberof circular regions with a diameter of 0.5 μm having no inorganicfillers is one or less and the number of circular regions with adiameter of 0.3 μm having no inorganic fillers is three or less in animage of a 5 μm square of said rubber composition determined by ascanning probe microscope (SPM).

In the rubber composition of the present invention, as the rubberypolymer there can be used, for example, diene polymers, olefin polymers,urethane polymers, and the like. Preferred rubbery polymers are dienepolymers and olefin polymers.

Diene polymers are, for example, at least one conjugated diene compound,or a copolymer of said conjugated diene compound and at least one othercopolymerizable monomer.

As conjugated diene compounds, there can be used, for example,1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene and the like, and asother copolymerizable monomers, there can be used, for example, styrene,α-methyl styrene, p-methyl styrene, divinylbenzene, 1,1-diphenylethylene, acrylonitrile, acrylic esters, methacrylic esters and thelike, with vinyl aromatic compounds being preferred.

As diene polymers, there can be used, for example, butadiene rubber(BR), isoprene rubber (IR), styrene-butadiene rubber (SBR),styrene-isoprene-butadiene rubber (SIBR), nitrile-butadiene rubber(NBR), further high purity natural rubber and the like. Furthermore, themodified polymers, branched polymers such as coupling, or hydrogenatedcompounds thereof can also be used. Monomer units in the copolymers maybe randomly distributed or may be distributed in blocks.

Olefin polymers may be obtained by polymerizing olefins, and encompassall of the rubbery polymers. For example, buthyl rubber (IIR),ethylene-propylene rubber (EPM.EPDM), ethylene alpha olefin copolymers,ethylene styrene copolymers, and the like. Furthermore,ethylene-conjugated diene copolymers may also be included.

Urethane polymers are, for example, urethane rubber (TPU).

The rubbery polymers for use in the present invention may be a linear orbranched structure.

Also, modified rubbery polymers in which the ends have been modified bymodifying agents are a preferred raw material rubber, and modifyingagents include, for example, metal compounds such as tin compounds,hydroxy group-containing compounds, carboxyl group-containing compounds,amine derivatives, amides, imines, silane compounds, epoxy compounds andthe like, and specifically there can be preferably used modifying agentscontaining modifying groups having high affinity for silica such ashydroxy group-containing compounds, amine derivatives, amides, imines,silane compounds, and epoxy compounds. Two or more of such modifyingagents may be used in combination, or other modifying agents may also beused.

Furthermore, the rubbery polymer of the present invention is composed ofpolymer component (A) and component (B), preferably having differentrefractive indexes. Component (A) has a refractive index of 1.535 orgreater and less than 1.600, preferably 1.540 or greater and less than1.600. Enhancement of the refractive index of a polymer may beaccomplished by increasing the amount of monomer components having highrefractive index such as vinyl aromatic compounds. Examples of vinylaromatic compounds having high refractive index include styrene,α-methyl styrene, p-methyl styrene, divinylbenzene, 1,1-diphenylethylene and the like.

Component (B) is a polymer having a refractive index of 1.400 or greaterand less than 1.530. Since the rubbery polymer comprises a polymer oftwo components having different refractive index, the haze becomedecreased and thereby a rubber composition having a good transparencycan be obtained.

Preferably the difference in refractive index between component (A) andcomponent (B) is 0.010 or greater, and more preferably 0.020 or greater.The greater the difference in refractive index is, the higher thetransparency of the rubber composition becomes.

The content of component (A) in the rubbery polymer is 5-80% by weight,preferably 10-75% by weight, and most preferably 15-70% by weight, andcomponent (B) is 20-95% by weight.

When component (A) has a composition of 5-80% by weight, a goodtransparency can be obtained, and when component (B) has a compositionof 20-95% by weight, balance of rubber properties such as abrasionresistance and c-set and transparency becomes favorable.

In the rubber composition of the present invention, a preferredcomponent (A) is a copolymer of a vinyl aromatic compound and aconjugated diene compound. Preferred vinyl aromatic compounds include,for example, styrene, α-methyl styrene, p-methyl styrene,divinylbenzene, and 1,1-diphenyl ethylene, with vinyl styrene mostpreferred.

As conjugated diene compounds, there can be used, for example,1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene and the like.1,3-butadiene and isoprene are preferred.

As other examples of monomers copolymerizable with conjugated dienecompounds, there can be mentioned acrylonitrile, acrylic esters,methacrylic esters, and the like.

When component (A) having a refractive index of 1.535 or greater andless than 1.600 of the rubbery polymer of the rubber composition of thepresent invention is a copolymer comprising a vinyl aromatic compoundand a conjugated diene compound, the amount of the vinyl aromaticcompound bound in said copolymer is preferably 26-90% by weight. Whenthe amount of the vinyl aromatic compound bound in said component (A)copolymer is 26-90% by weight, the rubber composition obtained has agood transparency, and thus has favorable gripping properties for use inshoe soles and the like. When it is 90% by weight or less, the mixingand processing properties of the rubber composition become excellent andfavorable dispersibility of the inorganic filler can be obtained.

Also, the binding structure of the vinyl aromatic compound and theconjugated diene compound in the component (A) copolymer may be a blockdistribution or a random distribution, and may be selected based on, butnot limited to, the purpose of the rubber composition obtained.

When component (B) having a refractive index of 1.400 or greater andless than 1.530 of the rubbery polymer of the rubber composition of thepresent invention is a diene polymer, the amount of the vinyl aromaticcompound bound is preferably 25% by weight or less. Since the amount ofthe vinyl aromatic compound bound in the diene polymer of component (B)is 25% by weight or smaller, an excellent transparency can be obtained.

The binding structure of the vinyl aromatic compound and of theconjugated diene compound in the diene polymer of component (B) may be,but not limited to, a block distribution or a random distribution.

It is preferred that the amount of the vinyl aromatic compound bound inthe rubbery polymer component (A) in the rubber composition of thepresent invention is greater than in component (B) by 25% by weight ormore, in which case rubber compositions having a more excellenttransparency can be obtained. The lower the compatibility of component(A) and component (B), the more excellent the transparency of the rubbercomposition to which an inorganic filler has been added becomes.

Component (A) specifically preferred for use as the rubbery polymer ofthe rubber composition of the present invention is a styrene-butadienecopolymer in which the amount of the styrene bound is 26-90% by weight,and more preferably the amount of the styrene bound is 30-80% by weight.When the amount of the styrene bound of component (A) is 26-90% byweight, a rubber composition having an excellent balance of transparencyand physical properties can be obtained.

As component (A), two or more types of styrene-butadiene copolymershaving different amounts of styrene bound can be used. The binding ofstyrene in the styrene-butadiene copolymer may be, but not limited to, ablock distribution or a random distribution. The distribution of thevinyl bound in the butadiene portion may also be a block distribution ora random distribution, but in order to satisfy abrasion resistance, theamount of the vinyl bound is preferably 80% by weight or less,preferably 60% by weight or less, and more preferably 40% by weight orless.

The binding of styrene of the styrene-butadiene copolymer having theamount of styrene bound of 25% by weight or less as a preferredcomponent (B) of the present invention can be, but not limited to, ablock distribution or a random distribution. The distribution of thevinyl bound in the butadiene portion may be, but not limited to, a blockdistribution or a random distribution. But in order to satisfy abrasionresistance, the amount of the vinyl bound is preferably 80% by weight orless, preferably 60% by weight or less, and more preferably 40% byweight or less. In the case of polybutadiene, in order to satisfyabrasion resistance, the amount of the vinyl bound is preferably 80% byweight or less, preferably 60% by weight or less, and more preferably40% by weight or less.

As component (B) for use in the present invention, highcis-polybutadiene in which 1,4-cis bond is 80% by weight or more isspecifically preferred. In that case, abrasion resistance becomes moreexcellent in addition to transparency.

According to the present invention, those component (A) and component(B) in which the acetone extract is 2% by weight or less arespecifically preferred. The components of the acetone extract, which arethe residue of emulsifying agents, acids, salts etc. used for emulsionpolymerization and the like, persist in rubber leading to an enhancedhaze and a reduced transparency of the rubber composition, and is alsoresponsible for coloring. Thus, the preferred rubbery polymers that isused in the present invention are those polymers having very fewimpurities produced in solution polymerization.

As inorganic fillers of the rubber composition of the present invention,there can be used, for example, silica, zinc flower, zinc carbonate,magnesium carbonate, magnesium hydroxide, calcium carbonate, magnesiumcarbonate, clay etc., and combinations of two or more of them can beused.

More preferred inorganic fillers are those that have refractive index inthe range of 1.450-1.570 from the viewpoint of conferring more favorabletransparency, and they include, for example, silica, magnesium carbonateand magnesium hydroxide. A specifically preferred inorganic filler issilica. As silica, there can be used those that are called dry silica,wet silica, synthetic silicate white carbon, colloidal silica, and thelike. Also, silica of which surface has been rendered hydrophobic, andmixtures of silica and an inorganic filler other than silica can beused, but most preferred is dry silica having a small particle size witha specific surface area of 100 m²/g or more. The mean primary particlediameter of dry silica is preferably 50 nm or smaller, more preferably40 nm or smaller, most preferably 30 nm or smaller. By selecting silicahaving a small primary particle diameter and then subjecting it to anoptimum kneading condition of the present invention, rubber compositionshaving a more preferred transparency can be obtained. It is preferredthat the primary silica particles are smaller, but when the dispersionstate is not good, secondary particles remain as they are, andaggregates larger than or as large as the visible light wavelength arepresent, and conversely regions in which there is no silica present areformed, and the difference of refractive indexes of those regionsproduce opaque rubber compositions. Since specific inorganic fillerssuch as silica are in an excellent dispersion state in rubbercompositions, the desired transparency can be attained.

According to the rubber composition of the present invention, the amountof inorganic fillers used is 5-150 parts by weight per 100 parts byweight of the raw material rubber, the rubbery polymer, preferably10-120 parts by weight, and most preferably 10-100 parts by weight.Within this range, there can be obtained rubber compositions having anexcellent transparency and an excellent balance with abrasion resistanceand c-set.

Thus, the transparent rubber composition of the present invention is arubber composition, wherein the haze is 50% or less, and the inorganicfiller is dispersed so that the number of circular regions with adiameter of 0.5 μm having no inorganic fillers is one or less and thenumber of circular regions with a diameter of 0.3 μm having no inorganicfillers is three or less in an image of a 5 μm square of said rubbercomposition determined by a scanning probe microscope (SPM). Morepreferable is a rubber composition having a more preferred haze of 37%or less, and an excellent dispersibility that the number of circularregions with a diameter of 0.5 μm having no inorganic fillers is zero,the number of circular regions with a diameter of 0.3 μm having noinorganic fillers is one or less, and the number of circular regionswith a diameter of 0.2 μm having no inorganic fillers is three or lessin an image of a 5 μm square of said rubber composition determined by ascanning probe microscope (SPM). By satisfying the haze anddispersibility of inorganic fillers, an excellent transparency andrubber physical properties of various molded products can be obtained.

The transparent rubber composition of the present invention observed bySPM is a rubber composition in which the mean dispersed particlediameter of the inorganic fillers is dispersed to the primary particlelevel of 50 nm or less, preferably 30 nm or less, and the particles takea homogeneous distribution state in which there are almost no aggregatenetwork of inorganic fillers. Specifically, in rubber compositionshaving blended therein inorganic filler with a potent aggregatingproperty such as silica, there can be usually observed, when observed bySPM, regions in which very few inorganic fillers are present and regionsin which they are densely distributed, but in the transparent rubbercompositions of the present invention having an excellent transparency,there are almost no regions in which inorganic fillers are not present,and they take a morphology in which inorganic fillers are evenlydispersed.

It is preferred that the rubbery polymers for use in the presentinvention are polymers having very few impurities, just as polymersproduced by solution polymerization, in order to obtain an excellenttransparency and coloring.

The Mooney viscosity of the rubbery polymer of the transparent rubbercompositions of the present invention is not specifically limited, and apreferred range of Mooney viscosity (ML1+4 100° C.) is 10-150, and inthe case of highly hard polymers it is in the range of 5-120 at thecondition of ML5+4 100° C. or MS1+4 100° C. or the like. Any range ofMooney viscosity may be used as long as it allows processing.

A non-contaminating oil can be added to the rubber composition of thepresent invention as a softening agent. Such an oil may be a naphtheneoil or a paraffin oil, and a preferred amount added is 50 parts byweight or less per 100 parts by weight of the raw material rubber.

Also, various additives may be added to the rubber composition of thepresent invention in order to improve the characteristics.

Additives may include, for example, silane coupling agents that linkinorganic fillers to rubber.

Silane coupling agents include, for example, alkoxysilane compounds suchas tetraethoxysilane, vinyl trimethoxysilane,vinyltris(2-methoxyethoxy)silane, 3-aminopropyl triethoxysilane,3-glycidoxypropylmethyl dimethoxysilane,bis-[3-(triethoxysilyl)-propyl]tetrasulfide,bis-[3-(triethoxysilyl)-propyl]disulfide,triethoxysilylpropyl-methacrylate-monosulfide and the like, and mostpreferably tetraethoxysilane, vinyl trimethoxysilane, andvinyltris(2-methoxyethoxy)silane. The preferred amount of silanecoupling agents used according to the present invention is 0-20 parts byweight per 100 parts by weight of raw material rubber, and mostpreferably 0.5-10 parts by weight, and two types or more may be used incombination. By using silane coupling agents, mechanical strength of therubber composition such as modulus, tensile strength, tear strength etc.can be increased, and c-set, abrasion resistance etc. are enhanced toprovide an excellent balance with transparency.

As additives, dispersing agents and activating agents may be added. Forexample, fatty acids such as stearic acid, oleic acid, and lauric acidor derivatives thereof, or diethylene glycol, polyethylene glycol etc.may be added at 5 parts by weight or less per 100 parts by weight of theraw material rubber, and those having as low melting point as possibleare more preferred. If the amounts of such dispersing agents andactivating agents added exceeds 5 parts by weight, turbidity andbleeding of the rubber compositions obtained may occur, which is notdesirable.

Furthermore, there can be added various stabilizers such asnon-contaminating age resistors, anti-oxidants, antiozonants, and UVabsorbers, and pigments depending on uses.

When the rubber compositions of the present invention are cross linkedor vulcanized, various crosslinking agents and vulcanizing agents may beused. They are, for example, sulfur, sulfur compounds, organicperoxides, oximes, nitroso compounds, polyamines, metal compounds andthe like, and they may be used in combination. Preferred are sulfur,sulfur compounds, and organic peroxides, with organic peroxides beingmost preferred such as t-butyl hydroperoxide, cumene hydroperoxide,dicumyl peroxide, di-t-butyl peroxide,2,5-dimethyl-2,5-(t-butylperoxy)hexane, benzoylperoxide, and1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane. In vulcanization ofthe sulfur series, use of non-contaminating vulcanization acceleratorsis preferred.

The method of producing the rubber composition of the present inventionis a method of producing rubber compositions comprising 100 parts byweight of at least one rubbery polymer and 5-150 parts by weight of atleast one inorganic filler per 100 parts by weight of said rubberypolymer, which comprises kneading said rubbery polymer and saidinorganic filler at least once for 30 seconds or longer at a rubbertemperature of 0-60° C. during the step of kneading.

Thus, the method of producing the rubber composition of the presentinvention is a method of producing rubber compositions, which compriseskneading a rubbery polymer and an inorganic filler at a kneadingtemperature of 0-60° C., preferably 10-50° C., for 30 seconds or longer,preferably one minute or longer, in the step of kneading, and bykneading within this range, the dispersibility of inorganic fillers isenhanced and balance of transparency with physical properties such asabrasion resistance or c-set becomes more excellent.

In such a method of producing rubber compositions according to thepresent invention, any type of kneading machines may be used. Forexample, a Banbury mixer, a pressure kneader, an internal mixer, anextruder, an open roll, and the like are cited.

The method of kneading rubber compositions according to the presentinvention may be a method in which at a temperature range of 0-60° C.,all or part of the rubbery polymer and an inorganic filler and, asneeded, all or part of other additives may be added and kneaded for 30seconds or longer, or a method in which kneading is conducted at a giventemperature of 60-180° C. due to the heat generated by kneading or thelike until a given time.

Alternatively, the method of producing the rubber composition of thepresent invention may be a method in which, at the first step, blendingis effected at a maximum kneading temperature of 60-180° C., and thenthe rubber composition is kneaded again at 0-60° C. for 30 seconds orlonger, and during re-kneading, a vulcanizing agent, a crosslinkingagent, or another additive may be added.

Furthermore, it may be a method of producing rubber compositions whichcomprises adding and kneading rubber and an inorganic filler and, asneeded, another additive using a mixing roll etc. at a constanttemperature range of 0-60° C., or a method of blending and kneadingrubber and an inorganic filler etc. at a high temperature exceeding 60°C. as in the conventional method, and blending them at a conventionalkneading temperature, followed by kneading again the rubber compositionat 0-60° C. for 30 seconds or longer, said method comprising adding,during re-kneading another additive such as a vulcanizing agent, acrosslinking agent etc.

Though a high temperature exceeding 60° C. is required since thekneading of a highly hard rubber is difficult at low temperature, ahighly hard rubber and a less hard kneadable rubber may be blended at atemperature 60° C. or higher and then rubber compositions may beproduced according to the production method of the present invention.

Alternatively, methods of producing the rubber compositions of thepresent invention may be producing rubber compositions which compriseskneading by repeating kneading at a high temperature and kneading at alow temperature such as a method of production comprising kneadingsimultaneously a material that requires processing at a hightemperature, another rubber, and other additives such as an inorganicfiller, and a vulcanizing agent at a temperature of 60° C. or higher,and then kneading again at a temperature of 0-60° C. or a method furthercomprising kneading them again at a temperature of 60° C. or higher, andother methods.

In brief, methods of producing the rubber compositions of the presentinvention are characterized in that a rubbery polymer and an inorganicfiller are kneaded at a temperature range of 0-60° C. for 30 seconds orlonger in either of the step of kneading continuously, the initial stepof kneading in multi-steps, a midway step, and the final step, andintend to enhance dispersibility of the inorganic filler in the rubbercomposition in order to enhance transparency and physical properties.

When moisture content etc. is high in the raw material rubber andinorganic fillers for use in the method of producing the rubbercomposition of the present invention, said moisture may aggravatetransparency or weaken the reinforcing effect leading to decreasedphysical properties. When using raw material rubber or inorganic fillershaving such a high moisture content, hopefully they are preferably driedprior to use. It is also possible to conduct kneading simultaneouslywith removal of moisture by evaporation at the high temperature kneadingat 100° C. or more. If there is residual water in the blending,crosslinking and vulcanizing become heterogeneous, turbidity and airbubbles may arise in the vulcanized rubber compositions obtained, andvarious physical properties become inadequate.

The present invention also relates to a footwear comprising a rubbercomposition having the haze of the rubber composition 5 mm thick is 50%or less, wherein the number of circular regions with a diameter of 0.5μm or more having no inorganic fillers is one or less and the number ofcircular regions with a diameter of 0.3 μm or more having no inorganicfillers is three or less in an image of a 5 μm square determined by ascanning probe microscope (SPM) of a rubber composition comprising 100parts by weight of at least one rubbery polymer and 5-150 parts byweight of at least one inorganic filler per 100 parts by weight of saidrubbery polymer. The rubber compositions of the present invention may beused in any applications making use of transparency, coloringcharacteristics, abrasion resistance, c-set and rubber elasticity,rubber hardness, and the like.

For example, they are specifically useful for use in shoe soles,sandals, and various parts of footwear, and may also be used in varioustires, automobile parts, OA equipment and stationeries, casters,household electrical appliances, medical uses, sports goods, toys,furniture, and the like.

Now, the present invention will be explained with reference to examples,but it should be noted that the scope of the present invention is notlimited to these examples in any way.

[Preparation of the Raw Material Rubbery Polymer]

A rubbery polymer which is to be a raw material of the rubbercomposition used in the Examples and Comparative Examples below wasprepared in a batch polymerization using a 10 L autoclave equipped witha stirrer that had been replaced with nitrogen according to thepolymerization procedure in the following Table 1.

The total amount of monomer was set at 1.00 kg, which was subjected tocopolymerization with a styrene monomer and 1,3-butadiene using n-butyllithium as a polymerization initiator in a cyclohexane solvent (550 phm)to yield styrene butadiene rubbers SBR-A to −F.

As a vinylation agent tetrahydrofuran was used except for SBR-E, and theamount of vinyl bound in butadiene was controlled.

For SBR-E ((4) in Table 1), potassium-tert-butyl alcholate was used at aratio of 1/30 moles relative to n-butyl lithium.

In each polymerization batch, the conversion rate (measured by gaschromatography) two minutes after reaching the maximum polymerizationtemperature was 99.5% or more for styrene, and 99.8% or more for1,3-butadiene. Thereafter, for SBR-A, tin tetrachloride was added at anamount described in (1) of Table 1, and for SRB-C, tetraglycydyl-1,3-bisaminomethyl cyclohexane was added to a portion of SRB-Bat an amount described in (2) of Table 1, and for SBR-D, 1,3-dimethylimidazolydinone was added at an amount described in (3) of Table 1 andwas subjected to coupling or denaturation reaction.

For SRB-F, 24 phr among 1,3-butadienes described in Table 1 according tothe random styrene-butadiene production method described inJP-A-59-140211 was added to the polymerization system continuously for 6minutes from midway of polymerization to the maximum polymerizationtemperature for randomization. TABLE 1 SBR-A SBR-B SBR-C SBR-D SBR-ESBR-F Styrene monomre 25 43 43 45 64 56 (parts by weight) 1,3-butadiene75 57 57 55 36 44 (parts by weight) n-butyl lithium 0.076 0.084 0.0840.081 0.90 0.068 (phm) Tetrahydrofuran 1.1 2.4 2.4 2.3 (4) 2.6 (phm)Polymerization 62 63 63 59 67 66 starting temperature (° C.) Maximum 90105 105 92 89 95 polymerization temperature (° C.) Coupling agent, (1)None (2) (3) None None modifying agent 0.80 1.20 1.00 (Li equivalents)

After the completion of reaction in each run, a 10-fold molar amount ofwater relative to n-butyl lithium was added to stop the reaction, and asa stabilizer 2,6-di-tert-butyl-4-methylphenol was added to the rubbersolution at an amount of 0.3 part by weight per 100 parts by weight ofthe rubber, and the solvent was evaporated and dried by a drum drier,and finished.

The following Table 2 shows the amount of styrene bound in the producedstyrene-butadiene rubber, the amount of styrene block, the amount ofvinyl bound in the bound butadiene, and Mooney viscosity.

The amount of styrene bound was determined using a UV spectrophotometer(manufactured by JASCO Corporation, V-520 UV) by comparing with the UVabsorption intensity of the standard polystyrene.

The amount of styrene block was determined by dissolving two parts byweight of the copolymer in 100 parts by weight of chloroform, to whichfive parts by weight of tertiary-butyl hydroperoxide and 0.01 part byweight of osmium tetraoxide were added, which was then decomposed understirring in a 80° C. water bath for 15 minutes, and to the decomposedsolution 10 volumes of methanol was added to filter the resultingprecipitate (filter mesh size: 0.45 μm), which was dried and from whichweight the amount of styrene block in the polymer was measured.

The amount of 1,2-vinyl bound in butadiene was determined using an IRspectrophotometer (manufactured by JASCO Corporation, V-520 UV)according to the Hampton technique. TABLE 2 Amount of styrene Amount ofAmount of Mooney Acetone bound (% styrene vinyl viscosity extract byblock (% bound (% (ML1 + 4 (% by weight by weight by weight 100° C.)weight SBR-A 25 8 19 57 0.7 SBR-B 43 16 28 43 1.0 SBR-C 43 15 28 61 0.9SBR-D 45 14 34 48 0.8 SBR-E 64 9 13  71* 1.2 SBR-F 56 1 35 36 0.9*Mooney viscosity of SBR-E was measured under the condition of ML5 + 4100° C.

The acetone extract was measured by placing a rubber sample sheet with athickness of 0.5 mm to a 200 mesh metal net sack, drying the sheet undervacuum after extraction under acetone reflux and measuring it from thedifference in weight before and after the procedure.

As other rubbers, the following rubbers were used.

SBR-G is JSR0202, and the acetone extract 4.1% (manufactured by JSRCorporation, the amount of styrene bound: 46% by weight, emulsionpolymerization SBR with Mooney viscosity ML1+4 100° C. being 45).

BR-A is D35R (manufactured by Asahi Kasei Corporation, low-cis BR withMooney viscosity ML1+4 100° C. being 35).

BR-B is UBEPOL-150 (manufactured by Ube Industries, Ltd., cis 1,4-bond98% by weight, BR with Mooney viscosity ML1+4 100° C. being 43).

[Measurement of Refractive Index of Various Rubbery Polymers]

A refractometer manufactured by ATAGO CO., LTD. was used and the indexwas measured at 23° C. according to the JIS-K7105.

The result of measurement was:

-   -   SBR-A: 1.538    -   SBR-B: 1.553    -   SBR-C, 1.553    -   SBR-D: 1.555    -   SBR-E: 1.574    -   SBR-F: 1.565    -   SBR-G: 1.557    -   BR-A: 1.519    -   BR-B: 1.523

For blending and kneading, a 6-inch open roll that permits temperaturecontrol and a 0.3 L kneader were used.

[Method of Crosslinking Rubber Compositions and of Determining PhysicalProperties of Vulcanized Products]

In the Examples and Comparative Examples below, the following equipmentand conditions were used for determination of physical properties ofeach rubber composition.

Measurement of Dispersibility of Inorganic Fillers

-   -   Scanning probe microscope (SPM) manufactured by Seiko        Instruments Inc., Model: SPI3800N    -   Measurement condition: room temperature (23° C.)    -   Cyclic contact mode, phase image

An image of 5 μm square was enlarged to 20,000 fold, in which circlescorresponding to diameters of 0.5 μm, 0.3 μm and 0.2 μm were drawn inregions having no inorganic fillers, and the numbers thereof werecounted.

Measurement of Transparency

Using a haze meter, type NDH-1001DP manufactured by Nippon DenshokuIndustries, Co., Ltd., was used to measure the haze and PT (paralleltransmission) of sheet samples having a thickness of 5 mm of rubbercompositions obtained in each of Examples and Comparative Examples.

Measurement of Compression-Set (C-Set) Resistance

It was measured according to JIS K6301 under the condition of 70° C., 22hours.

Measurement of Abrasion Resistance

Using an Akron abrasion machine, abrasion loss with a load of 6 poundsand 3000 rpm was measured.

Measurement of the Hardness of Vulcanized Rubber

It was measured using a Shore A hardness meter (23° C.).

Others

For representative Examples, pull tests at 23° C. and measurement ofRupke impact resilience at 23° C. were conducted according to JIS-K6301.

EXAMPLE 1

As the raw material rubber, 70 g of SBR-B and 130 g of BR-B were wrappedand mixed by a mixing roll at 35° C., and 70 g of silica-1 (ReorosilQS20, manufactured by Tokuyama Corporation specific surface area 220m²/g), 2 g of an age resistor (NOCRAC SP) and 4 g of a silane couplingagent (Vinyl trimethoxysilane) were added and kneaded at the same rolltemperature over about 5 minutes. Then, 1.0 g of dicumyl peroxide wasadded thereto at the same roll temperature (rubber temperature 46° C.),and kneaded again for 2 minutes to prepare a rubber composition.

COMPARATIVE EXAMPLE 1

As the raw material rubber, 70 g of SBR-B and 130 g of BR-B were wrappedand mixed by a mixing roll at 100° C., and 70 g of silica-1 (ReorosilQS20), 2 g of an age resistor (Nocrac SP) and 4 g of a silane couplingagent (Vinyl trimethoxysilane) were added and kneaded at the same rolltemperature over about 5 minutes (rubber temperature 103° C.). Then, 1.0g of dicumyl peroxide was added thereto at the same roll temperature,and kneaded again for 1 minute to prepare a rubber composition.

EXAMPLE 2

As the raw material rubber, 70 g of SBR-A and 130 g of BR-B were wrappedand mixed by a mixing roll at 100° C., and 60 g of silica-1 (ReorosilQS20), 2 g of an age resistor (NOCRAC SP) and 4 g of a silane couplingagent (Vinyl trimethoxysilane) were added and kneaded at the same rolltemperature over about 5 minutes (rubber temperature 104° C.). Aftercooling to room temperature, 1.0 g of dicumyl peroxide was added theretoat a roll temperature of 30° C., and kneaded again for 2 minutes (rubbertemperature 43° C.) to prepare a rubber composition.

EXAMPLE 3

As the raw material rubber, 80 g of SBR-G and 120 g of BR-B were wrappedand mixed by a mixing roll at 35° C., and 70 g of silica-1 (ReorosilQS20), 2 g of an age resistor (NOCRAC SP) and 4 g of a silane couplingagent (Vinyl trimethoxysilane) were added and kneaded at the same rolltemperature over about 5 minutes (rubber temperature 49° C.). Then, 1.0g of dicumyl peroxide was added thereto at the same roll temperature,and kneaded again for 3 minutes to prepare a rubber composition.

EXAMPLE 4

As the raw material rubber, 70 g of SBR-C and 130 g of BR-B were wrappedand mixed by a mixing roll at 35° C., and 80 g of silica-1 (ReorosilQS20), 2 g of an age resistor (NOCRAC SP) and 4 g of a silane couplingagent (Vinyl trimethoxysilane) were added and kneaded at the same rolltemperature over about 5 minutes (rubber temperature 43° C.). Then, 1.0g of dicumyl peroxide was added thereto, and kneaded again for 2 minutesto prepare a rubber composition.

EXAMPLE 5

As the raw material rubber, 60 g of SBR-F and 140 g of BR-A were wrappedand mixed by a mixing roll at 40° C., and 70 g of silica-1 (ReorosilQS20), 2 g of an age resistor (NOCRAC SP) and 4 g of a silane couplingagent (Vinyl trimethoxysilane) were added and kneaded at the same rolltemperature over about 7 minutes (rubber temperature 49° C.). Then, 1.0g of dicumyl peroxide was added thereto, and kneaded again for 2 minutesto prepare a rubber composition.

EXAMPLE 6

As the raw material rubber, 150 g of SBR-B and 15 g of silica-2 (NipsilVN3 previously dried for 2 hours in a 120° C. oven), 1.5 g of an ageresistor (NOCRAC SP) and 3 g of a silane coupling agent (Vinyltrimethoxysilane) were added in a 0.3 L kneader, and kneading wasstarted at a cylinder indication temperature of 30° C. Thirty secondslater the temperature reached 37° C. and 15 g of silica-2 was furtheradded and kneading was continued. 15 g of the rest of silica-2 was addedthereto at an indication temperature of 47° C. (2 minutes after thestart). Seven minutes later, the rubber blend was removed at anindication temperature of 118° C. 30 g of the rubber blend dischargedfrom the kneader was molded by press molding into a sheet 5 mm thick,and the haze thereof was measured. The haze was 43%. Then, 0.64 g ofdicumyl peroxide was added thereto at a 50° C. roll, and kneaded for 2minutes to prepare a rubber composition.

COMPARATIVE EXAMPLE 2

As the raw material rubber, 150 g of SBR-B and 15 g of silica-1(Reorosil QS20), 1.5 g of an age resistor (NOCRAC SP) and 3 g of asilane coupling agent (Vinyl trimethoxysilane) were added in a 0.3 Lkneader, and kneading was started at a cylinder indication temperatureof 75° C. Thirty seconds later the indication temperature reached 91°C., and 15 g of silica-1 was added thereto and kneading was continued,and at an indication temperature of 105° C. (2 minutes after the start)the rest of 15 g of silica-1 was added. 5 minutes and 30 seconds later,the indication temperature reached 129° C. and the rubber blend wasremoved. 30 g of the rubber blend discharged from the kneader was moldedby press molding into a sheet 5 mm thick, and the haze thereof wasmeasured. The haze was 75%. Then, 0.64 g of dicumyl peroxide was addedthereto at a 80° C. roll, and kneaded to prepare a rubber composition.

EXAMPLE 7

As the raw material rubber, 15 g of SBR-A, 45 g of SBR-D, 90 g of BR-B,20 g of silica-1 (Reorosil QS20), 1.5 g of an age resistor (NOCRAC SP)and 3 g of a silane coupling agent (Vinyl trimethoxysilane) were addedin a 0.3 L kneader, and kneading was started at a cylinder indicationtemperature of 33° C. 20 g of silica-1 (Reorosil QS20) was added theretoat an indication temperature of 38° C. Kneading was further continued,and at 1 minute 20 seconds, the indication temperature reached 50° C.and the rest of 20 g of silica-1 (Reorosil QS20) was added. Kneading wascontinued, and five minutes later the rubber blend was removed at anindication temperature of 121° C. Then, 0.64 g of dicumyl peroxide wasadded thereto at a 50° C. roll, and kneaded to prepare a rubbercomposition.

COMPARATIVE EXAMPLE 3

As the raw material rubber, 60 g of SBR-B and 140 g of BR-B were wrappedand mixed by a mixing roll at 35° C., and 6 g of silica-1 (ReorosilQS20), 2 g of an age resistor (NOCRAC SP) and 4 g of a silane couplingagent (Vinyl trimethoxysilane) were added and kneaded at the same rolltemperature over about five minutes (rubber temperature 43° C.). Then,1.0 g of dicumyl peroxide was added thereto at the same rolltemperature, and kneaded again for 1 minute to prepare a rubbercomposition.

The composition of the rubber compositions prepared in Examples 1-7 aresummarized in the following Table 3, and the composition of the rubbercompositions prepared in Comparative Examples 1-3 are summarized in thefollowing Table 4. TABLE 3 (Unit: parts by weight) Ex. 1 Ex. 2 Ex. 3 Ex.4 Ex. 5 Ex. 6 Ex. 7 SBR-A 35 10 SBR-B 35 100 SBR-C 35 SBR-D 30 SBR-F 30SBR-G 40 BR-A 70 BR-B 65 65 60 65 60 Silica-1 35 30 35 40 35 40 Silica-230 vinyl 2 2 2 2 2 2 2 trimethoxysilane NOCRAC SP 1 1 1 1 1 1 1 Dicumylperoxide 0.5 0.5 0.5 0.5 0.5 0.43 0.43

In the table, silica-1 was Reorosil QS20 manufactured by TokuyamaCorporation and silica-2 was Nipsil VN3 manufactured by Nippon SilicaIndustries., Co. NOCRAC SP was manufactured by Ouchi Shinko ChemicalIndustrial Co., Ltd. TABLE 4 (Unit: parts by weight) Com. Ex. 1 Com. Ex.2 Com. Ex. 3 SBR-A SBR-B 35 100 30 SBR-C SBR-D SBR-F SBR-G BR-A BR-B 6570 Silica-1 35 3 Silica-2 30 vinyl 2 2 2 trimethoxysilane NOCRAC SP 1 11 Dicumyl peroxide 0.5 0.43 0.5In the table, silica-1 was Reorosil QS20 manufactured by TokuyamaCorporation and silica-2 was Nipsil VN3 manufactured by Nippon SilicaIndustries., Co. NOCRAC SP was manufactured by Ouchi Shinko ChemicalIndustrial Co., Ltd.

The rubber compositions obtained in the above Examples 1-7 andComparative Examples 1-3 were press vulcanized at 160° C. for 10 minutes(15 MPa pressure), and after allowing to stand all day and night tocool, physical properties such as the transparency, Akron abrasion andc-set etc. of the vulcanized rubber compositions were measured. Theresults on Examples are shown in Table 5, and those on ComparativeExamples are shown in Table 6. TABLE 5 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex.6 Ex. 7 *The number of regions having no inorganic fillers Regiondiameter of 0.5 μm or 0 0 0 0 0 0 0 greater Region diameter of 0.3 μm or0 2 0 0 0 1 0 greater Region diameter of 0.2 μm or 0 3 1 0 2 4 0 greaterHaze (%) 19 43 45 17 29 39 26 Parallel transmission (%) 63 34 31 65 5443 58 c-set(%) 20 19 27 15 21 22 17 Akron abrasion CC/3000 times 0.0260.025 0.026 0.018 0.029 0.034 0.021*The number of regions having no inorganic fillers are the number ofregions with a diameter of 0.5 μm or greater, the number of regions witha diameter of 0.3 μm or greater, and the number of regions with adiameter of 0.2 μm or greater in the SPM image.

TABLE 6 Com. Ex. 1 Com. Ex. 2 Com. Ex. 3 *The number of regions havingno inorganic fillers Region diameter of 0 0 1 0.5 μm or greater Regiondiameter of 6 5 2 0.3 μm or greater Region diameter 0.2 μm 11 9 18 orgreater Haze (%) 77 73 72 Parallel 19 15 16 transmission (%) c-set (%)25 27 22 Akron abrasion 0.046 0.053 0.114 CC/3000 times*The number of regions having no inorganic fillers are the number ofregions with a diameter of 0.5 μm or greater, the number of regions witha diameter of 0.3 μm or greater, and the number of regions with adiameter of 0.2 μm or greater in the SPM image.

With regard to silica dispersibility by the confirmation of the SPMimage, there were very few circular regions having no silica with adiameter of 0.3 μm or greater in the rubber compositions of Examples1-7. In contrast, there were many regions having no silica with adiameter of 0.3 μm or greater in the rubber compositions of ComparativeExamples 1-3, and the haze was high and transparency was poor.

In Example 1, the rubber temperature at kneading was 60° C. or lower,and the abrasion resistance and c-set of the crosslinked product weregood and had a remarkably excellent transparency as compared toComparative Example 1 in which kneading was carried out at a rolltemperature of 100° C. The hardness of the crosslinked product ofExample 1 had sufficient rubber characteristics with a Shore A of 64, a300% modulus of 10.6 Mp, a tensile strength of 19.2 Mp, an extension of510%, a tear strength of 41 N/mm, and a Rupke impact resilience of 52 ata room temperature (23° C.).

In Example 2 in which the first step was a high temperature kneading at100° C. and at the second step kneading was carried out at a lowtemperature, transparency and balance of physical properties were moreexcellent than that of Comparative Example 1.

Example 3 exhibited a little coloring at yellow, and the haze was notgreater than 50%.

Example 4 was an example of a modified styrene-butadiene rubber, whichhad an excellent transparency and good balance with c-set.

In Example 6, a wet type silica was dried prior to use in the blendingby a kneader. At a temperature of 60° C. or less, a styrene-butadienerubber and silica were kneaded for 2 minutes or more, and then thetemperature started to rise resulting in a high-temperature kneading,but by kneading again at 50° C., transparency and balance of physicalproperties were more excellent than that of Comparative Example 1 orComparative Example 2.

In Example 7, two types of styrene-butadiene rubbers were used, buttransparency and balance of physical properties were excellent.

In Comparative Example 3, the amount of silica was too little andtransparency was poor, and there was very little reinforcing effect ofthe filler leading to poor abrasion resistance.

EXAMPLE 8

As the raw material rubber, 70 g of SBR-B and 130 g of BR-B were wrappedand mixed by a mixing roll at 65° C., and 140 g of silica-1 (ReorosilQS20), 2 g of an age resistor (NOCRAC SP) and 4 g of a silane couplingagent (Vinyl trimethoxysilane) were added and kneaded at the same rolltemperature over about 12 minutes, and after cooling at roomtemperature, 1.0 g of dicumyl peroxide was added thereto at a rolltemperature of 30° C., and kneaded again for 2 minutes to prepare arubber composition.

EXAMPLE 9

As the raw material rubber, a given amount described in Table 3 of theraw material rubber that was prepared by mixing 40 g of SBR-B and 40 gof SBR-E in a mixing roll at 80° C., and then after cooling at roomtemperature, premixed at a mixing roll temperature of 35° C., 120 g ofBR-B and 80 g of silica-1 (Reorosil QS20), 2 g of an age resistor(NOCRAC SP) and 4 g of a silane coupling agent (Vinyl trimethoxysilane)were added and kneaded for 6 minutes, and then under the sametemperature condition (rubber temperature 43° C.), 1.0 g of dicumylperoxide was added thereto, and kneaded again for 2 minutes to prepare arubber composition.

EXAMPLE 10

At a mixing roll temperature of 70° C., 20 g of SBR-D, 40 g of SBR-E,140 g of BR-B, 100 g of silica-1 (Reorosil QS20), 2 g of an age resistor(NOCRAC SP) and 4 g of a silane coupling agent (Vinyl trimethoxysilane)were added and kneaded for 7 minutes, and after cooling at roomtemperature, 1.0 g of dicumyl peroxide was added thereto at a mixingroll temperature of 35° C. (rubber temperature 43° C.), and kneadedagain for 5 minutes to prepare a rubber composition.

COMPARATIVE EXAMPLE 4

At a mixing roll temperature of 70° C., 20 g of SBR-B, 40 g of SBR-E,140 g of BR-B, 100 g of silica-1 (Reorosil QS20), 2 g of an age resistor(NOCRAC SP) and 4 g of a silane coupling agent (Vinyl trimethoxysilane)were added and kneaded for 7 minutes, and then 1.0 g of dicumyl peroxidewas added and kneaded for 2 minutes to prepare a rubber composition.

The composition of the rubber compositions prepared in Examples 8-10 andComparative Example 4 are summarized in Table 7. TABLE 7 (Unit: parts byweight) Ex. 8 Ex. 9 Ex. 10 Com. Ex. 4 SBR-A SBR-B 35 20 10 SBR-C SBR-D10 SBR-E 20 20 20 BR-B 65 60 70 70 Silica-1 70 40 50 50 Vinyl 2 2 2 2trimethoxysilane NOCRAC SP 1 1 1 1 Dicumyl peroxide 0.5 0.5 0.5 0.5In the table, silica-1 was Reorosil QS20 manufactured by TokuyamaCorporation and NOCRAC SP was manufactured by Ouchi Shinko ChemicalIndustrial Co., Ltd.

The rubber compositions obtained in the above Examples 8, 9, and 10 andComparative Example 4 were press vulcanized at 160° C. for 10 minutes(15 MPa pressure), and after allowing to stand all day and night to coolin a thermostatic chamber, the transparency and hardness of thevulcanized rubber compositions were measured. The results are shown inTable 8. TABLE 8 Ex. 8 Ex. 9 Ex. 10 Com. Ex. 4 *The number of regionshaving no inorganic fillers Region diameter 0 0 0 0 of 0.5 μm or greaterRegion diameter 0 0 0 4 of 0.3 μm or greater Region diameter 0 0 2 8 of0.2 μm or greater Haze (%) 17 28 26 86 Parallel 64 58 59 11 transmission(%) Hardness Shore A 96 93 92 93*The number of regions having no inorganic fillers are the number ofregions with a diameter of 0.5 μm or greater, the number of regions witha diameter of 0.3 μm or greater, and the number of regions with adiameter of 0.2 μm or greater in the SPM image.

By increasing the amount of silica in Example 8 and adding astyrene-butadiene copolymer having a great amount of styrene bound inExample 9 and 10, rubber compositions having a high hardness and anexcellent transparency were obtained. The rubber composition ofComparative Example 4 has a high hardness, but the haze is high andtransparency is poor.

INDUSTRIAL APPLICABILITY

Since the rubber compositions of the present invention have an excellentbalance of transparency and physical properties such as c-set, and havean excellent transparency even when converted to highly hard products,they can be used in various applications that require these physicalproperties of rubber in addition to transparency and beautiful coloring,and are specifically useful in applications for footwear.

1. A rubber composition comprising 100 parts by weight of a rubberypolymer and 5-150 parts by weight of an inorganic filler per 100 partsby weight of said rubbery polymer, wherein the haze of said rubbercomposition having a thickness of 5 mm is 50% or less, and the inorganicfiller is dispersed so that the number of circular regions with adiameter of 0.5 μm or more having no inorganic fillers is one or lessand the number of circular regions with a diameter of 0.3 μm or morehaving no inorganic fillers is three or less in an image of a 5 μmsquare of said rubber composition determined by a scanning probemicroscope (SPM).
 2. The rubbery composition according to claim 1wherein said rubbery polymer comprises 5-80% by weight of a rubberypolymer component (A) having a refractive index of 1.535 or greater andless than 1.600 and 20-95% by weight of a rubber polymer component (B)having a refractive index of 1.400 or greater and less than 1.530. 3.The rubber composition according to claim 2 wherein the difference inrefractive index between component (A) and component (B) is 0.010 orgreater.
 4. The rubber composition according to claim 2 or 3 whereincomponent (A) is a copolymer of a vinyl aromatic compound and aconjugated diene compound in which the amount of the vinyl aromaticcompound bound is 26-90% by weight, and component (B) is a diene polymerin which the amount of the vinyl aromatic compound bound is 25% byweight or less, and the haze of said rubber composition having athickness of 5 mm is 37% or less.
 5. The rubber composition according toclaim 2 or 3 wherein the amount of the vinyl aromatic compound bound inthe copolymer of component (A) is greater than that in the polymer ofcomponent (B) by 25% by weight or more.
 6. The rubber compositionaccording to claim 2 or 3 wherein the acetone extract in each ofcomponent (A) and component (B) is 2% by weight or less.
 7. The rubbercomposition according to claim 2 or 3 wherein the component (B) is apolybutadiene in which the amount of 1,4-cis bond is 80% by weight ormore.
 8. The rubber composition according to any one of claims 1 to 3wherein the inorganic filler is a silica filler having a specificsurface area of 100 m²/g or more.
 9. The rubber composition according toany one of claims 1 to 3 wherein the inorganic filler is a dry silicahaving a mean primary particle diameter of 30 mm or smaller.
 10. Amethod of producing a rubber composition comprising 100 parts by weightof a rubbery polymer and 5-150 parts by weight of an inorganic fillerper 100 parts by weight of said rubbery polymer, wherein the haze ofsaid rubber composition having a thickness of 5 mm is 50% or less, andthe inorganic filler is dispersed so that the number of circular regionswith a diameter of 0.5 μm or more having no inorganic fillers is one orless and the number of circular regions with a diameter of 0.3 μm ormore having no inorganic fillers is three or less in an image of a 5 μmsquare of said rubber composition determined by a scanning probemicroscope (SPM), said method comprising kneading said rubbery polymerand said inorganic filler for at least 30 seconds or longer per run ofkneading at a rubber temperature of 0-60° C.
 11. The method of producinga rubber composition according to claim 10 wherein the rubbercomposition comprises 5-80% by weight of a rubbery polymer component (A)having a refractive index of 1.535 or greater and less than 1.600 and20-95% by weight of a rubbery polymer component (B) having a refractiveindex of 1.400 or greater and less than 1.530, and said rubbery polymerand the inorganic filler are kneaded for at least 30 seconds or longerper run of kneading at a rubber temperature of 10-50° C.
 12. A footwearcomprising the rubber composition according to any one of claims 1 to 3.